1. Field of the Invention
The present invention relates to thermotropic liquid crystalline polymers reinforced with wollastonite, and more particularly, to the production of molding compounds and molded or extruded articles produced from such reinforced thermotropic liquid crystalline polymers.
2. Description of the Prior Art
Many thermotropic liquid crystalline polymers have poor surface characteristics, particularly with respect to abrasion. One attempt to reduce such abrasion has been the incorporation of various fillers and reinforcing agents.
For example, the use of various fillers is disclosed in commonly-assigned U.S. Pat. No. 4,067,852 to Calundann. Calundann teaches that fillers and/or reinforcing agents may be included in a total concentration of about 1 to 60% by weight of the resulting molding compound. Representative fibers which may serve as reinforcing media therein include glass fibers, asbestos, graphitic carbon fibers, amorphous carbon fibers, synthetic polymeric fibers, aluminum fibers, aluminum silicate fibers, oxide of aluminum fibers, titanium fibers, magnesium fibers, rock wool fibers, steel fibers, tungsten fibers, cotton wool, and wood cellulose fibers, etc. If desired, the fibrous reinforcement may be preliminarily treated to improve its adhesion ability to the liquid crystalline polymer which ultimately serves as a continuous matrix phase. Representative filler materials disclosed in Calundann include calcium silicate, silica, clays, talc, mica, polytetrafluoroethylene, graphite, aluminum trihydrate, sodium aluminum carbonate, barium ferrite, etc.
U.S. Pat. No. 4,267,304 to Feasey et al discloses the use of reinforcing fillers such as glass fibers, asbestos, aluminum silicate fibers, and synthetic polymer fibers, as well as the use of particulate fillers such as kaolin, chalk, silica, and glass ballottini.
Thus, the incorporation of various fillers and reinforcements into liquid crystalline polymers has been investigated as shown above. However, although the surface abrasion can be reduced by blending mineral fillers such as talc and mica into liquid crystalline polymer, mechanical properties such as tensile strength and impact strength are significantly reduced in such blends. Although reinforcements such as carbon fiber also reduce surface abrasion, they are much more expensive than mineral fillers.
Another filler which is not mentioned in any of the above patents is wollastonite. Wollastonite is a naturally occurring calcium silicate found in metamorphic rocks having the chemical formula CaSiO.sub.3. The reference in Calundann to calcium silicate is not equivalent to wollastonite since the term "calcium silicate" does not specify the chemical stoichiometry (CaSiO.sub.3, Ca.sub.2 SiO.sub.4, and Ca.sub.3 SiO.sub.5, for example, are all calcium silicates). Furthermore, commercially prepared calcium silicate could be a powder, while wollastonite particles are acicular; that is, they are shaped like small needles or spines. Thus, the wollastonite can function as a fibrous reinforcing agent rather than just as a filler.
The use of wollastonite in combination with liquid crystalline polymers is disclosed in European Patent Application No. 81/302836.2 of Bailey et al. Bailey et al describe the use of inert fillers at greater than 10% by volume and preferably higher than 20% by volume, to obtain a reduction in anisotropy ratio. The inert fillers in Bailey et al include particulate or fibrous fillers which do not react with the polymer of the composition but may have been surface treated to make them more compatible with the polymer matrix. The preferred class of fillers is glass fibers in the form of short fibers (one specific example uses three millimeter glass fibers) or glass mats. Although wollastonite is used in one example at a concentration of 70% by weight (53.7% by volume), no information is given concerning the size or aspect ratio of the wollastonite. By "aspect ratio" is meant the ratio of length to diameter of the fibers.
Accordingly, a need exists for a thermotropic liquid crystalline polymer blend which is less subject to surface abrasion without a concomitant large reduction in mechanical properties, such as tensile strength and impact strength. Furthermore, the reinforcing agent should be relatively inexpensive so as to not increase the overall cost of articles made from the polymer blend.